From the client side there really are no alternatives. From the content provider side there are 2 significant alternatives: Silverlight and HTML5. And the sooner everybody moves to them (probably mostly to 5) the sooner we all can uninstall Flash.
Microsoft Silverlight is a piece of crap!!!!!!!!!!!!!! It does not work. I can’t even play netflix videos with it. If I go to the test page then that works... But to used it in the actual world it does not work.
I have never installed Flash on any computer and had it not work.